The present invention relates to an internal combustion engine of the in-cylinder fuel injection type in which gasoline is directly injected into a combustion chamber of the engine, and more particularly to a fuel injection valve of the electromagnetic type for directly injecting fuel into the combustion chamber.
Among the various types of internal combustion engines, a conventionally widely used gasoline engine has fuel injected into an air-intake port thereof using an electromagnetic valve. Intake air and the injected fuel are mixed inside the air-intake port, and the mixed gas is then supplied to and is ignited and burned in a combustion chamber of the engine to generate power.
On the other hand, in a diesel engine, which mainly uses light oil as a fuel, among internal combustion engines, fuel is directly injected into a combustion chamber of the engine, and the fuel is burned in spontaneous combustion by compressed air inside the combustion chamber to generate power. Thus, in the type of gasoline engine described above, an in-cylinder fuel injection directly into the combustion chamber of the engine has been proposed in order to improve the response of the internal combustion engine.
In such a gasoline engine of in-cylinder fuel injection type, since a spark plug is provided at the top of the combustion chamber as an ignition means in contrast to a diesel engine, the space available for location of the electromagnetic fuel Injection valve is limited. That is, if the temperature of a position near the exhaust valve becomes high, the fuel injection valve cannot be placed there. On the other hand, since the air-intake valve side is less affected by the combustion gas though there is an intake passage there, it has been proposed that the electromagnetic fuel injection valve be installed near the air-intake valve.
Even if the electromagnetic fuel injection valve is installed near the air-intake valve in the cylinder head, the temperature of the cylinder head of the gasoline engine reaches 250 to 300.degree. C. during operation of the engine. Therefore, the end portion of the electromagnetic fuel injection valve is also heated up to near the same temperature. Under such a high temperature condition, the production and accumulation of deposits of carbonization-growing materials in the injected gasoline and a mixture of engine lubricant and the gasoline is accelerated. Most of the deposits are amorphous flakes. However, when the temperature is increased, the carbonization progresses further to form soot with a grain structure having a size of several tens of nano-meters. The deposits and the soot attach to and accumulate on the wall surface of the combustion chamber and the exposed portion of the electromagnetic fuel injection valve.
In order to solve the problem of accumulation of deposits and soot, Japanese Patent Application Laid-Open No. 3-225068 proposes that an externally opening valve be employed, and ring-shaped projections projecting outward are provided in the peripheral portion of the end of the valve head and in the peripheral portion of an opening of the fuel injection nozzle of the valve body. By doing so, appropriate fuel injection is performed under conditions wherein the projections concentratively receive heat from the engine as compared to the other parts of the valve and the temperature of the projection is locally increased to suppress formation of deposits due to attached fuel drops.
Japanese Patent Application Laid-Open No. 6-147022 discloses a fuel injection device for directly injecting fuel into a combustion chamber in which exhaust gas is recirculated in order to decrease NOx exhaust.
In general, the deposits and the soot which attach onto the wall surface of the combustion chamber are not considered a problem since they increase the thermal insulating efficiency so as to improve the combustion efficiency. However, when deposits become attached to and accumulate at the valve portion of the electromagnetic fuel injection valve, particularly near the fuel injection nozzle and at an inside portion of the fuel injection nozzle provided at the end portion of the electromagnetic fuel injection valve, the injecting direction and the fuel spray shape are changed and the fuel mist particle size becomes large. Therefore, the engine operation is affected not only by a decreased combustion efficiency, but the fuel injection nozzle also becomes choked and the injection resistance is increased. As a result, a desired amount of fuel cannot be injected. In a worst case, the fuel injection nozzle becomes completely blocked so that fuel cannot be injected at all, and consequently the engine will not run.
Therefore, in the case of Japanese Patent Application Laid-Open No. 3-225068 described above, it is an objective to form a hot spot, which is locally increased to a high) temperature, to decrease the generation of deposits and soot. However, since the heat capacity of the hot spot is small, the temperature is rapidly decreased during the intake stroke and deposits are generated at that time, which deposits are carbonized in the next combustion stroke. Therefore, there is a possibility that the desired effect cannot always be obtained.
On the other hand, in the case of Japanese Patent Application Laid-Open No. 6-147022 described above, although Nox exhaust is reduced, the effect of deposits and soot which become attached near the injection nozzle of the fuel injection valve is not sufficiently considered, and so there is a possibility that such deposits will cause degradation in the performance of the fuel injection valve.
The inventors of the present invention have experimentally studied an in-cylinder fuel injection device which is capable of use in a gasoline engine while taking the results of the conventional technology into consideration. As a result, the following facts have been revealed.
(1) When the spread angle of fuel spray particles injected from a fuel injection valve is narrow, the penetrating force of the fuel spray, that is, the kinetic energy in the injecting direction becomes large. However, when the fuel spray particles reach an inner wall surface of the cylinder facing the fuel spray, the fuel spray particles form a film thereon. The fuel film takes a long time to evaporate, and so a part of the fuel does not completely evaporate, but remains to form a mixture having a dense fuel concentration, thereby resulting in a red-flame burning to create a lot of soot.
(2) On the other hand, when the spread angle of fuel spray particles is sufficiently wide, the penetrating force of the fuel spray becomes weak. Further, the fuel spray particles hit on a portion of the air-intake valve projecting into the cylinder, and so a part of the fuel spray particles hit the inner wall surface of the cylinder head, so as to again form a film on the cylinder wall in as the same manner as described in the item (1) above. The fuel spray particles which become attached onto the portion of the air-intake valve are re-atomized by the intake air flowing into the cylinder to moderate the bad effect on combustion. The fuel which becomes attached onto the cylinder head, on the other hand, burns with a red-flame since it takes a long time to evaporate for the same reasons as described in the item (1) above.
(3) A spark plug is arranged near the central portion of the cylinder head, and when fuel attaches to the spark plug, normal generation of a spark is not possible and consequently combustion cannot be started.
From the above results, in order to properly operate an engine over a wide speed range from starting to a rated speed state after starting, it is preferable that the spread angle of fuel spray particles is set to 60.degree. to 90.degree.. Further, it has been found that there is an appropriate range in the spread angle of fuel spray particles depending on the operating condition of the engine. For example, it is preferable to produce an optimum fuel injection condition where the spread angle of the fuel spray particles is suppressed to a small value, namely nearly 60.degree., to form a good flammable condition when the temperature of the wall surface is low, such as at starting time, and to widen the spread angle of the fuel spray particles, namely to nearly 90.degree., to disperse the mixture of fuel and air and to decrease the amount of soot being generated by suppressing local red-flame burning, when the combustion has progressed and the temperature of the wall surface becomes high.
Various shapes are proposed for the nozzle of an electromagnetic fuel injection valve for producing fuel spray particles. The inventors of the present invention have recognized that freedom in the forming of a fuel spray is increased by giving a swirl to the fuel on the upstream side of a single hole orifice, and that the combustion results described above can be obtained by adjusting the spread angle of the fuel spray particles by giving a swirl to the fuel on the upstream side of a single hole orifice. on the other hand, in an electromagnetic fuel injection valve of this type, deposits and soot attach onto the injection valve itself. The reason is that the temperature of the surface of the fuel injection valve is heated up to 170.degree. C. to 200.degree. C. by heat directly received from combustion and by heat conduction from the cylinder head, and it is estimated that the end portion of the fuel injection valve reaches an even higher temperature, since it is exposed to the combustion gas.